Hydrostatically operated continuously variable transmission

ABSTRACT

A hydrostatically operated continuously variable transmission includes a cylinder block centrally coupled to a transmission shaft coupled to an engine, a swash-plate-type axial-piston hydraulic pump having a group of pump plungers, a hydraulic motor having a group of motor plungers, and a closed hydraulic circuit by which the hydraulic pump and the hydraulic motor are interconnected. The groups of pump and motor plungers are disposed in annular patterns in the cylinder block, and disposed concentrically such that one of the groups surrounds the other group. The plunger groups are positionally displaced in the circumferential direction of the cylinder block such that a circle inscribed in said one group passes through the other group.

BACKGROUND OF THE INVENTION

The present invention relates to a hydrostatically operated continuouslyvariable transmission including a swash-plate-type axial-pistonhydraulic pump and a hydraulic motor which are coupled in a closedhydraulic circuit, the hydraulic pump and motor having respective groupsof pump and motor plungers which are disposed in annular patterns in acommon cylinder block that is centrally coupled to a transmission shaft.

Hydrostatically operated continuously variable transmissions are knownin the art as disclosed in Japanese Laid-Open Patent Publication No.61-153057, for example.

In the conventional hydrostatically operated continuously variabletransmission, groups of pump and motor plungers are disposed parallel tothe axis of a cylinder block, and hence the cylinder block whichaccommodates these pump and motor plungers must be axially elongate.Therefore, it has been difficult to make the hydrostatically operatedcontinuously variable transmission more compact.

A swash-plate-type hydraulic device, which may serve as theswash-plate-type axial-piston hydraulic pump or the hydraulic motor, hasa working oil distributor having mutually concentric high- andlow-pressure hydraulic chambers defined in the cylinder block around theaxis thereof. The working oil distributor includes a number of radialdistribution valves that are reciprocally movable between radially outerand inner positions in the cylinder block for bringing cylinder holesdefined in the cylinder block into alternate communication with thehigh- and low-pressure hydraulic chambers. An eccentric ring is disposedeccentrically with respect to the center of rotation of the cylinderblock in contact with the radially outward portions of the distributionrings for imparting reciprocating movement to the distribution rings inresponse to relative rotation of the cylinder block and the plungerswash plate. Such a working oil distributor is known from JapaneseLaid-Open Patent Publication No. 61-153057.

The swash-plate-type hydraulic device tends to have an axial dimensiongreater than the outside diameter of the cylinder block since annulargroups of plungers disposed in the cylinder holes are disposed parallelto the axis of the cylinder blocks. With the conventional working oildistributor incorporated in the swash-plate-type hydraulic device, thegroup of distribution valves must be located at one end of the group ofplungers, resulting in an increase in the axial dimension of thecylinder block, and it is difficult to render the swash-plate-typehydraulic device more compact.

SUMMARY OF THE INVENTION

In view of the aforesaid problems of the conventional hydrostaticallyoperated continuously variable transmission, it is an object of thepresent invention to provide a hydrostatically operated continuouslyvariable transmission of a compact configuration with groups of pump andmotor plungers being housed in a cylinder having a reduced axialdimension.

Another object of the present invention is to provide a hydrostaticallyoperated continuously variable transmission having a swash-plate-typehydraulic device of a compact size including a working oil distributorwhich comprises distribution valves arranged without involving anincrease in the axial dimension of a cylinder block.

According to the present invention, there is provided a hydrostaticallyoperated continuously variable transmission for use with an engine,comprising a transmission shaft adapted to be coupled to the engine, acylinder block centrally coupled to the transmission shaft, aswash-plate-type axial-piston hydraulic pump having a group of pumpplungers, a hydraulic motor having a group of motor plungers, and aclosed hydraulic circuit by which the hydraulic pump and the hydraulicmotor are interconnected, the groups of pump and motor plungers beingdisposed in annular patterns in the cylider block, the groups of pumpand motor plungers being disposed concentrically such that one of thegroups surrounds the other group, and being positionally displaced inthe circumferential direction of the cylinder block such that a circleinscribed in said one group passes through the other group.

The pump plungers may be disposed inwardly of the motor plungers or thevice versa.

Since the pump plungers and the motor plungers are disposedconcentrically, the axial dimension of the cylinder block whichaccommodates the pump and motor plungers is greatly reduced.

Inasmuch as the inner and outer plunger group are positionally displacedwith respect to each other in the circumferential direction of thecylinder block such that the circle inscribed in the outer plunger grouppasses through the inner plunger group, the portion of the cylinderwhich lies between the inner and outer plungers has a sufficient wallthickness but the inner and outer plunger groups are disposed closely toeach other. Therefore, any increase in the diameter of the cylinderblock arising from the concentric arrangement of the plunger groups isminimized.

According to the present invention, there is also provided a working oildistributor in a swash-plate type hydraulic device, comprising acylinder block having an annular array of cylinder holes and a pluralityof plungers slidably disposed respectively in the cylinder holes, aplunger swash plate engaging projecting ends of the plungers, high- andlow-pressure oil chambers defined in the cylinder block for supplyingworking oil to and receiving working oil from the cylinder holes inresponse to relative rotation of the cylinder block and the plungerswash plate, the high- and low-pressure oil chambers being disposedannularly around the axis of the cylinder block and arranged axially ofthe cylinder block, a plurality of distribution valves disposedannularly in the cylinder block concentrically with the plungers andreciprocally movable between positions spaced axially of the cylinderblock for alternately communicating the cylinder holes with the high-and low-pressure oil chambers, a holder by which the plunger swash plateis supported, and a valve swash plate contacting ends of thedistribution valves for imparting reciprocating motion to thedistribution valves in response to relative rotation of the cylinderblock and the plunger swash plate, the valve swash plate being supportedon the holder obliquely to the axis of the cylinder block.

The working oil distributor may be incorporated in a hydraulic pump or ahydraulic motor.

When the cylinder block and the plunger swash plate are relativelyrotated, the distribution valves are caused by the valve swash plate tomove reciprocally in the axial direction of the cylinder block toalternately communicate the corresponding cylinder holes with thehigh-and low-pressure oil chambers for supplying and receiving workingoil.

Since the distribution valves are disposed in an annular patternconcentric with the plungers, the cylinder block with the distributionvalves is not increased in axial dimension.

The valve swash plate may be positioned in a space around the plungerswash plate. Therefore, the valve swash plate does not result in anincrease in the axial dimension of the hydraulic device.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an automotive power unitincorporating a hydrostatically operated continuously variabletransmission according to the present invention;

FIG. 2 is a longitudinal cross-sectional view of the hydrostaticallyoperated continuously variable transmission shown in FIG. 1;

FIG. 3 is a cross-sectional view taken along line III--III of FIG. 2;

FIG. 4 is a cross-sectional view taken along line IV--IV of FIG. 3;

FIG. 4A is a view similar to FIG. 4, showing operation of thetransmission;

FIG. 5 is a circumferentially developed cross-sectional view taken alongline V--V of FIG. 2;

FIG. 6 is a cross-sectional view taken along line VI--VI of FIG. 3;

FIG. 7 is a fragmentary cross-sectional view taken along line VII--VIIof FIG. 2;

FIG. 8 is a fragmentary cross-sectional view taken along line VIII--VIIIof FIG. 2;

FIG. 9 is an enlarged fragmentary cross-sectional view of a clutch valvewhich is shown as being in a partly engaged position;

FIG. 10 is a cross-sectional view taken along line X--X of FIG. 3;

FIG. 11 is a view as viewed in the direction of the arrow XI in FIG. 3;and

FIG. 12 is a graph showing the relationship between the angles ofinclination of pump and motor swash plates and the transmission ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, an automotive power unit U generally comprises anengine E, a hydrostatically operated continuously variable transmissionT according to the present invention, and a differential Df which areall housed and supported in a casing C serving as a stationary machineframe.

The engine E has a crankshaft 1, and the continuously variabletransmission T positioned at the righthand (FIG. 1) end of the engine Ehas an input shaft 2 disposed as a transmission shaft coaxially with thecrankshaft 1 and coupled thereto through a torque damper Td. Thecontinuously variable transmission T has an output gear 3 locatedclosely to the engine E and meshing with a ring gear 4 of thedifferential Df. The differential Df has axially opposite output shafts5, 5' parallel to the crankshaft 1 and the input shaft 2 for drivingrespective road wheel axles (not shown).

The input shaft 2 has a righthand end projecting out of the casing C andsupporting a pulley 6 thereon for driving various accessories such as apower steering hydraulic pump, an air conditioning compressor, and thelike.

As shown in FIGS. 2 and 3, the continuously variable transmission Tgenerally comprises a swash-plate-type axial-piston hydraulic pump P ofthe variable displacement type and a swash-plate-type axial-pistonhydraulic motor M of the fixed displacement type. The principles of thepresent invention are applied particularly to a working oil distributorin the hydraulic motor M.

The hydraulic pump P has a cylindrical cylinder block 7 having an oddnumber of cylinder bores or holes 8 defined therein in an annularpattern around the center of rotation of the cylinder block 7, thecylinder holes 8 having open righthand ends, a number of pump plungers 9slidably disposed in the cylinder holes 8, respectively, a pump swashplate 10 held against the outer or righthand ends of the pump plungers9, a trunnion shaft 12 of a segmental cross section having a flatsurface supporting the back of the pump swash plate 10 through a thrustroller bearing 11, and a swash plate anchor 13 in which the partlycylindrical surface of the trunnion shaft 12 is rotatably supported.

The swash plate anchor 13 is fastened to the casing C by means of a bolt15.

In order to allow the trunnion shaft 12 to rotate through a prescribedangle and also to prevent axial movement of the trunnion shaft 12, abolt 23 (FIGS. 3 and 11) is fixed to one end surface of the trunnionshaft 12 through an arcuate slot 22 defined in the swash plate anchor 13about the axis 01 of the trunnion shaft 12.

The pump swash plate 10 is tiltable between an upright position in whichit is normal to the axis of the cylinder block 7 and two maximum tiltedpositions on both sides of the upright position, in which it is inclinedat a certain angle, in response to rotation of the trunnion shaft 12.When the pump swash plate 10 is tilted, the pump plungers 9 can bereciprocally moved in repeated suction and discharge strokes in responseto rotation of the cylinder block 7.

The hydraulic motor M has as many cylinder bores or holes 18 defined inthe cylinder block 7 as the number of the cylinder holes 8 in an annularpattern concentric with and on a pitch circle slightly larger indiameter than the pitch circle of the cylinder holes 8, the cylinderholes 18 having open lefthand ends, a plurality of motor plungers 19slidably disposed in the cylinder holes 18, respectively, a motor swashplate 20 held against the outer or lefthand ends of the motor plungers19, and a motor swash plate holder 27 supporting the back of the motorswash plate 20 through a thrust roller bearing 26. The output gear 3 isformed on the outer periphery of the motor swash plate holder 27.

The inner cylinder holes 8 and the outer cylinder holes 18 arepositionally displaced relatively by half of the pitch of these cylinderholes in the circumferential direction of the cylinder block 7. A circleinscribed in the outer annular group of cylinder holes 18 passes throughthe inner annular group of cylinder holes 8.

The motor swash plate 20 is kept by the motor swash plate holder 27 soas to be tilted at an angle with respect to the axis of the cylinderblock 7 about an imaginary trunnion axis 02 normal to the axis of thecylinder block 7. When the cylinder block 7 and the motor swash plateholder 27 rotate relatively to each other, the motor swash plate 20reciprocally moves the motor plunger 19 in repeated expansion andcontraction strokes.

The input shaft 2 extending centrally through the cylinder block 7 iscoupled thereto by means of splines 32. The righthand end portion of theinput shaft 2 extends through the pump swash plate 10, the trunnionshaft 12, and the swash plate anchor 13. A thrust roller bearing 37 isinterposed between a first thrust support plate 35 mounted on therighthand end portion of the input shaft 2 and the swash plate anchor13. The righthand end portion of the input shaft 2 is rotatablysupported in the swash plate anchor 13 by a needle bearing 38 and in thecasing C by a bushing 39.

The first thrust support plate 35 is coupled to the input shaft 2 bymeans of a key 43.

The input shaft 2 has a lefthand end portion extending through the motorswash plate 20 and the motor swash plate holder 27. A thrust rollerbearing 40 is inteposed between a second thrust support plate 36 mountedon the lefthand end portion of the input shaft 2 and the motor swashplate holder 27, with a needle bearing 41 disposed between the motorswash plate holder 27 and the input shaft 2. The lefthand end portion ofthe input shaft 2 and the second thrust support plate 36 are rotatablysupported in the casing C by a roller bearing 42.

The swash plates 10, 20 have respective partly spherical recesses 10a,20a in which partly spherical ends 9a, 19a of the plungers 9, 19 engage,respectively, so as to allow the swash plates 10, 20 to rotate insynchronism with the cylinder block 7.

The hydraulic pump P and the hydraulic motor M are interconnected by aclosed hydraulic circuit as follows:

The cylinder block 7 has an annular inner oil chamber 45 defined in itsinner peripheral portion and first, second, and third annular outer oilchambers 46, 47, 48 defined in its outer peipheral portion and arrangedsuccessively axially from right to left as shown in FIGS. 2 and 3. Theinner oil chamber 45 and the third outer oil chamber 48 are held incommunication with each other through a plurality of radial oil passages49 (see FIG. 8).

The cylinder block 7 also has as many first valve holes 51 (see FIG. 4)as the number of the cylinder holes 8, the first valve holes 51extending radially adjacent to the closed ends of the cylinder holes 8,and as many second valve holes 52 as the number of the cylinder holes18, the second valve holes 52 extending axially adjacent to the cylinderholes 18 and being disposed radially outwardly thereof.

The first valve holes 51 extend from the outer peripheral surface of thecylinder block 7 to the inner oil chamber 45. Pump ports a (see FIG. 3)are defined in the cylinder block 7 respectively through the closed endsof the cylinder holes 8 and open into the respective valve holes 51 attheir intermediate portions. Oil passages 53 are defined in the cylinderblock 7 radially outwardly of the pump ports a, respectively, incommunication with the first outer oil chamber 46 (see FIGS. 3 and 6).

First spool-shaped distribution valves 55 are slidably disposedrespectively in the first valve holes 51. The radially outer ends of thefirst distribution valves 55 are engaged by a surrounding eccentric ring57 through a ball bearing 58. To keep the outer ends of the firstdistribution valves 55 in forced engagement with the eccentric ring 57,the outer ends of the first distribution valves 55 are interconnected bya forcing ring 59 concentric with the eccentric ring 57.

As shown in FIGS. 2, 4, and 4A, the eccentric ring 57 is coupled to theswash plate anchor 13 by a pivot shaft 60 parallel to the input shaft 2so as to be swingable between two postions, i.e., a hydraulicallyoperating position n shown in FIG. 4 and a lock-up position 1 shown inFIG. 4A. The eccentric ring 57 is controlled in its eccentricitysubstantially along the trunnion axis 01 with the center of the inputshaft 2 being used as a reference, by a control device (not shown)engaging a lever 57a projecting radially outwardly from the outersurface of the eccentric ring 57 remotely from the pivot shaft 60. Theamount of eccentricity of the eccentric ring 57 is ε when it is in thehydraulically operating position n and is zero (i.e., it is concentricwith the input shaft 2) when it is in the lock-up position 1.

When the eccentric ring 57 takes the hydraulically operating position nsee FIG. 4), and when the cylinder block 7 is rotated, the firstdistribution valves 55 are caused by the eccentric ring 57 toreciprocally move in the respective first valve holes 51 betweenradially inner and outer positions in the cylinder block 7 over a strokewhich is twice the amount of eccentricity ε. The hydraulic pump P is nowgiven a discharge region D and a suction region S.

In the discharge region D, as the cylinder block 7 rotates, the firstdistribution valves 55 move near the radially inner position to providecommunication between the corresponding pump ports a and the outer oilpassages 53 communicating with the first outer oil chamber 46 and tokeep those pump ports a out of communication with the inner oil chamber45, so that the pump plungers 9 in the discharge stroke dischargeworking oil from the cylinder holes 8 via the oil passages 53 into thefirst outer oil chamber 46. In the suction region S, the firstdistribution valves 55 move near the radially outer position to providecommunication between the corresponding pump ports a and the inner oilchamber 45 and to keep those pump ports a out of communication with theoil passages 53, so that the pump plungers 9 in the suction stroke drawworking oil from the inner oil chamber 45 into the cylinder holes 8.

Between the discharge and suction regions D, S, there are providedswitching neutral regions in which the pump ports a are held out ofcommunication with the inner oil chamber 45 and the outer oil passages53 by the first distribution valves 55.

When the eccentric ring 57 assumes the lock-up position 1 (see FIG. 4A),all of the first distribution valves 55 are kept in the switchingneutral position by the eccentric ring 57 irrespective of rotation ofthe cylinder block 7, thus closing all of the pump ports a.

As shown in FIGS. 2 and 3, the second valve holes 52 extend in thecylinder block 7 through the first, second, and third outer oil chambers46, 47, 48. Motor ports b are defined in the cylinder block 7 and openinto the respective valve holes 52 intermediate between the first andthird outer oil chambers 46, 48, the motor ports b communicating withthe cylinder holes 18 adjacent thereto (see FIGS. 2 and 7).

Second spool-shaped distribution valves 56 are slidably disposedrespectively in the second valve holes 52. The second distributionvalves 56 have ends normally engaging in partly spherical recesses 62aof a valve swash plate 62 supported on the motor swash plate holder 27through a thrust roller bearing 61 under the resiliency of springs 63.

The valve swash plate 62 is tilted a certain angle with respect to theaxis of the cylinder block 7 about a second imaginary trunnion axis 03which is 90° angularly spaced from or out of phase with the imaginarytrunnion axis 02 of the motor swash plate 20 about the axis of thecylinder block 7. When the cylinder block 7 and the motor swash plateholder 27 rotate relatively to each other, the valve swash plate 62reciprocally moves the second distribution valves 56 prescribed strokes.The positions of the second distribution valves 56 determine expansionand contraction regions Ex, Sh for the hydraulic motor M.

More specifically, as illustrated in FIG. 5, in the expansion region Ex,the second distribution valves 56 are in or near the lefthand limitposition to provide communication between the corresponding motor portsb and the first outer oil chamber 46 and to keep those motor ports b outof communication with the second outer oil chamber 47, so thathigh-pressure working oil is supplied from the first outer oil chamber46 into the cylinder holes 18 housing the motor plungers 19 in theexpansion stroke. In the contraction region Sh, the second distributionvalves 56 are in or near the righthand limit position to providecommunication between the corresponding motor ports b and the secondouter oil chamber 47 and to keep those motor ports b out ofcommunication with the first outer oil chamber 46, so that the motorplungers 19 in the contraction stroke discharge working oil from thecylinder holes 18 into the lower-pressure second outer oil chamber 47.

The second distribution valves 56 do not prevent communication betweenthe second and third outer oil chambers 47, 48 through the second valveholes 52. Therefore, oil discharged into the second outer oil chamber 47flows back into the inner oil chamber 45 via the third outer oil chamber48 and the oil passages 49.

Operation of the hydrostatically operated continuously variabletransmission T thus constructed is as follows: While the pump swashplate 10 is inclined at an angle and the eccentric ring 57 is kept inthe hydraulically operating position, the input shaft 2 is rotated aboutits own axis by the power of the engine E. In the hydraulic pump P,while the pump plungers 9 in the discharge stroke are passing throughthe discharge region D, they discharge working oil under pressure fromtheir cylinder holes 8 into the first outer oil chamber 46, whereas thepump plungers 9 in the suction stroke draw working oil from the inneroil chamber 45 into the cylinder holes 8 while those pump plungers 9 arepassing through the suction region S.

The high-pressure working oil discharged into the first outer oilchamber 46 is supplied into the cylinder holes 18 housing the motorplungers 19 in the expansion region Ex, while at the same time workingoil is discharged into the inner oil chamber 45 from the cylinder holes18 housing the motor plungers 19 in the contraction region Sh.

The output gear 3 is rotated by the sum of the torque applied by thecylinder block 7 via the motor plungers 19 to the motor swash plate 20and the torque applied by the motor plungers 19 in the expansion stroketo the motor swash plate 20. The torque of the output gear 3 is thentransmitted to the differential Df. It is now assumed that

n_(p) . . . the number of the pump plungers 9,

n_(M) . . . the number of the motor plungers 19,

d_(p) . . . the diameter of the pump plungers 9,

d_(M) . . . the diameter of the motor plungers 19,

D_(p) . . . the diameter of the pitch circle of the circular array ofthe pump plungers 9,

D_(M) . . . the diameter of the pitch circle of the circular array ofthe motor plungers 19,

O_(p) . . . the angle of inclination of the pump swash plate 10 to avertical plane,

O_(M) . . . the angle of inclination of the motor swash plate 20 to avertical plane,

S_(p) . . . the stroke of the pump plungers 9,

S_(M) . . . the stroke of the motor plungers 19,

Ni . . . the speed of rotation of the input shaft 2, and

No . . . the speed of rotation of the output gear 3.

The transmission ratio π can now be calculated by the followingequation: ##EQU1##

Where the motor swash plate 20 is angularly fixed at a certain angleθ_(M) as in the illustrated embodiment, therefore, if the angle θ_(p) ofinclination of the pump swash plate 10 is selected to be zero, then π≈1,or Ni≈No, so that the input shaft 2 and the output gear 3 are directlycoupled to each other

If θP<0, then π>1, i.e., Ni<No, and hence the speed of rotation of theoutput gear 3 is increased. ##EQU2## then Ni>No>0, and hence the speedof rotation of the output gear 3 is increased. ##EQU3## then No=0irrespective of Ni, and the transmission is in the neutral position.##EQU4## then No<0, and the transmission is in the reverse position.

The above conditions are illustrated in FIG. 12.

Even when the input shaft 2 and the output gear 3 are directly coupled,oil leakage is inevitable along the sliding surfaces subjected to highoil pressure, such as the sliding surfaces of the plungers 9, 19 and thecylinder holes 8, 18, and the sliding surfaces of the distributionvalves 55, 56 and the valve holes 51, 52, and hence the transmissionefficiency is lowered. And the condition No=Ni is often reached whilethe automobile is being driven. Under this condition, therefore, theeccentric ring 57 is controlled to move to the lock-up position 1 tocause all of the first distribution valves 55 to close all of the pumpports a. Since the hydraulic pump P and the hydraulic motor M are nowout of mutual communication, those sliding surfaces which are subjectedto high oil pressure are greatly reduced, and any reduction in thetransmission efficiency due to oil leakage is lowered.

At the time of operation of the hydraulic pump P and the hydraulic motorM, the pump swash plate 10 receives a thrust load from the pump plungers9 in one direction, and the motor swash plate 20 receives a thrust loadfrom the motor plungers 19 in the opposite direction. The thrust loadapplied to the pump swash plate 10 is transmitted to the input shaft 2through the thrust roller bearing 11, the trunnion shaft 12, the swashplate anchor 13, the thrust roller bearing 37, and the first thrustsupport plate 35. The thrust load applied to the motor swash plate 20 istransmitted also to the input shaft 2 through the thrust roller bearing26, the motor swash plate holder 27, the thrust roller bearing 40, andthe second thrust support plate 36. Therefore, the above thrust loadsonly develop tensile stresses in the input shaft 2, but are not imposedon the casing C.

As described above, the pump plungers 9 and the motor plungers 19 arepositionally displaced relatively in the circumferential direction ofthe cylinder block 7, and the circle inscribed in the annular array ofthe motor plungers 19 passes through the pump plungers 9. Therefore, thepump plungers 9 and the motor plungers 19 are positioned closely to eachother while providing partitions of enough thickness between theadjacent cylinder holes 8, 18. As a result, the axial dimension of thecylinder block 7 is greatly reduced while slightly increasing thediameter of the cylinder block 7.

This arrangement is highly effective in making compact the power unit Uin which the engine E and the transmission T are axially arranged.

By employing the axially reciprocable second distribution valves 56, thesecond distribution valves 56 can be disposed in position while slightlyincreasing the diameter of the cylinder block 7, and the axial dimensionof the cylinder block 7 can further be reduced. Since the valve swashplate 62 for actuating the second distribution valves 56 is disposed insurrounding relation to the motor swash plate 20, the valve swash plate62 does not contribute to any increase in the axial dimension of thetransmission T.

As shown in FIGS. 2 and 6, one or more third valve holes 65 (three valveholes 65 in the illustrated embodiment) are defined in the cylinderblock 7, the third valve holes 65 extending between the second valveholes 52 and parallel thereto. The third valve holes 65 extend acrossthe first and second outer oil chambers 46, 47 and have inner endsclosed and outer ends opening at the righthand end surface (FIG. 2) ofthe cylinder block 7. Spool-shaped clutch valves 66 are slidablydisposed respectively in the valve holes 65.

Each of the clutch valves 66 has an annular groove 67 defined in theouter periphery of its valve head projecting out from the righthand endsurface of the cylinder block 7. In the annular groove 67, there engagesa finger 68a bent radially inwardly from an inner operating ring 68slidably fitted over the cylinder block 7. An outer operating ring 69surrounding the inner operating ring 68 is coupled thereto by means of arelease bearing 70. A rack 71 projects from one end of the outeroperating ring 69 and extends axially thereof in mesh with a pinion 72which is rotatably supported in the swash plate anchor 13. A clutchlever 73 is fixed to one end of the pinion 72.

By turning the clutch lever 73 to rotate the pinion 72 for therebymoving the outer operating ring 69 horizontally (FIG. 2), each of theclutch valves 66 can be shifted through the release bearing 70 and theinner operating ring 68 between a lefthand clutch-on position (indicatedby the solid lines in FIG. 2) and a righthand clutch-off position(indicated by the two-dot-and-dash lines in FIG. 2). In the clutch-onposition, the clutch valves 66 cut off communication between the firstand second outer oil chambers 46, 47. In the clutch-off position, theclutch valves 66 allow communication between the first and second outeroil chambers 46, 47. When the first and second outer oil chambers 46, 47communicate with each other, high-pressure working oil supplied from thehydraulic pump P via the first distribution valves 55 to the first outeroil chamber 46 immediately flows into the lower-pressure second outeroil chamber 47, thus bypassing the hydraulic motor M. As a consequence,oil pressure transmission from the hydraulic pump P to the hydraulicmotor M is cut off in the clutch-off position.

As shown in FIG. 9, each of the clutch valves 66 has a land 66a forselectively cutting off communication between the oil chambers 46, 47and has one or more recesses 74 defined in an end thereof between theoil chambers 46, 47. When the clutch valve 66 is shifted from theclutch-off position to the clutch-on position, the recesses 74 providedelicate control over communication between the oil chambers 46, 47 forthereby reaching a partly engaging clutch position.

As illustrated in FIGS. 2 and 3, the input shaft 2 has a main oilpassage 75 defined centrally axially therein and having a closed end.The main oil passage 75 has an open end communicating with an oilreservoir 77 on the bottom of the casing C through a replenishing pump76. The replenishing pump 76 is actuated by a driver gear 78 on theouter periphery of the first thrust support plate 35 on the input shaft2. Therefore, as long as the input shaft 2 rotates, oil in the oilreservoir 77 is fed to the main oil passage 75 by the replenishing pump76.

The main oil passage 75 communicates with the inner oil chamber 45through first radial replenishing holes 79 defined in the input shaft 2.First check valves 81 (FIG. 2) are disposed respectively in the firstreplenishing holes 79 for preventing oil from flowing back from theinner oil chamber 45 into the main oil passage 75. The check valves 81are normally urged in a valve closing direction by a leaf spring 83surrounding the input shaft 2.

The main oil passage 75 is also held in communication with the firstouter oil chamber 46 through a second replenishing hole 80 defined inthe input shaft 2 and the cylinder block 7 and extending generallyradially. The second replenishing hole 80 accommodates therein a secondcheck valve 82 for preventing oil from flowing back from the first outeroil chamber 46 into the main oil passage 75. The check valve 82 isnormally urged in a valve closing direction by a valve spring 84.

During normal operation of the transmission T in which the hydraulicmotor M is hydraulically driven by the hydraulic pump P, working oil issupplied from the main oil passage 75 via the first replenishing oilpassages 79 into the lower-pressure inner oil chamber 45. During enginebraking in which the hydraulic pump P is hydraulically driven by thehydraulic motor M, working oil is supplied from the main oil passage 75via the second replenishing passage 80 into the first outer oil chamber46 which has become lower in pressure for thereby compensating forworking oil leakage from the closed hydraulic circuit.

As shown in FIGS. 3 and 10, a transmission control device 85 is coupledto the trunnion shaft 12 for controlling the angle of the pump swashplate 10. The transmission control device 85 comprises a sector gear 88fixed to the other end of the trunnion shaft 12 by means of a bolt 86and a pair of knock pins 87, a worm gear 89 meshing with the sector gear88, and a reversible DC motor 91 having a drive shaft 90 coupled to theworm gear 89. The worm gear 89 is rotatably supported in a gear box 93by means of bearings 94, 95, the gear box 93 being fixed to the casing Cby a bolt 92. The motor 91 has a stator fixed to a suitable portion ofthe casing C.

When the motor 91 is rotated in one direction or the other, the rotationis transmitted from the worm gear 89 to the sector gear 88 while itsspeed is being reduced thereby, and is then transmitted to the trunnionshaft 12 to rotate the same for erecting or tilting the pump swash plate10.

When the motor 91 is de-energized to keep the pump swash plate 10 at adesired angle, the pump swash plate 10 receives a moment in a directionto erect or tilt the same from the motor plungers 19, and such a momentis applied via the trunnion shaft 12 to the sector gear 88. However,since the worm gear 89 cannot be rotated by the sector gear 88, thegears 88, 89 are locked together to prevent the trunnion shaft 12 frombeing rotated under such a moment. Therefore, the pump swash plate 10 isreliably held in position when the motor 91 is de-energized.

In the illustrated embodiment, the hydraulic motor serves as a hydraulicdevice and the motor swash plate as a plunger swash plate. The first andsecond outer oil chambers 46, 47 serve as high- and low-pressure oilchambers, respectively.

The transmission T may be arranged such that the input shaft is used asthe output shaft and the output gear as the input gear with thehydraulic pump P serving as the hydraulic motor and the hydraulic motorM as the hydraulic pump.

With the arrangement of the present invention, as described above, thegroup of pump plungers and the group of motor plungers areconcentrically positioned such that one of the plunger groups surroundsthe other plunger group, and the plunger groups are positionallydisplaced relatively in the circumferential direction of the cylinderblock so that the circle inscribed in one of the plunger groups passesthrough the other plunger group. Consequently, the axial dimension ofthe cylinder block is greatly reduced, and any increase in the diameterof the cylinder block due to the concentrically disposed plunger groupsis minimized. As a result, the overall transmission is rendered compact.

Moreover, the high- and low-pressure oil chambers are defined annularlyaround the axis of the cylinder block and arranged axially of thecylinder block. The distribution valves are disposed annularly in thecylinder block in concentric relation to the plunger groups, thedistribution valves being reciprocally movable between positions spacedaxially of the cylinder block for alternately communicating the cylinderholes with the high-and low-pressure oil chambers. The valve swash platewhich contacts the ends of the distribution valves for impartingreciprocating motion to the distribution valves in response to relativerotation of the cylinder block and the plunger swash plate is supportedon the holder which supports the plunger swash plate obliquely to theaxis of the cylinder block. Thus, the distribution valves are disposedwithout increasing the axial dimension of the cylinder block. The valveswash plate does not result in an increase in the axial dimension of thehydraulic device. Therefore, the hydraulic device is made compactespecially in the axial direction thereof.

Although a certain preferred embodiment has been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

What is claimed is:
 1. A hydrostatically operated continuously variabletransmission for use with an engine, comprising:a transmission shaftadapted to be coupled to the engine; a cylinder block centrally coupledto said transmission shaft; a swash-plate-type axial-piston hydraulicpump having a group of pump plungers; a hydraulic motor having a groupof motor plungers; a closed hydraulic circuit by which said hydraulicpump and said hydraulic motor are interconnected; said groups of pumpand motor plungers being disposed in annular patterns in said cylinderblock; said groups of pump and motor plungers being disposedconcentrically such that one of said groups surrounds the other group,and being positionally displaced in the circumferential direction ofsaid cylinder block such that a circle inscribed in said one grouppasses through said other group; and a plurality of distribution valvesdisposed annularly in said cylinder block and aligned to reciprocatealong a direction parallel to the axis of said cylinder block fordistributing working oil between said pump and said motor.
 2. Ahydrostatically operated continuously variable transmission according toclaim 1, wherein said one group comprises the group of motor plungersdisposed in an annular pattern and said other group comprises the groupof pump plungers disposed in an annular pattern.
 3. A hydrostaticallyoperated continuously variable transmission according to claim 2,wherein said groups of pump and motor plungers are positionallydisplaced relatively to each other by half of the pitch of the pump andmotor plungers.
 4. A working oil distributor in a swash-plate typehydraulic device, comprising:a cylinder block having an annular array ofcylinder holes and a plurality of plungers slidably disposedrespectively in said cylinder holes; a plunger swash plate engagingprojecting ends of said plungers; high- and low-pressure oil chambersdefined in said cylinder block for supplying working oil to andreceiving working oil from said cylinder holes in response to relativerotation of said cylinder block and said plunger swash plate; said high-and low-pressure oil chambers being disposed annularly around the axisof said cylinder block and arranged axially of said cylinder block; aplurality of distribution valves disposed annularly in said cylinderblock concentrically with said plungers and reciprocally movable betweenpositions spaced axially of said cylinder block for alternatelycommunicating said cylinder holes with said high- and low-pressure oilchambers; a holder by which said plunger swash plate is supported; and avalve swash plate contacting ends of said distribution valves forimparting reciprocating motion to the distribution valves in response torelative rotation of said cylinder block and said plunger swash plate,said valve swash plate being supported on said holder obliquely to theaxis of said cylinder block.
 5. A working oil distributor according toclaim 4, wherein said plungers comprise a plurality of hydraulic motorplungers and said plunger swash plate comprises a hydraulic motor swashplate.
 6. A hydrostatically operated continuously variable transmissionfor use with an engine, comprising:a transmission shaft adapted to becoupled to the engine; a cylinder block centrally coupled to saidtransmission shaft; a swash-plate-type axial-piston hydraulic pumphaving a group of pump plungers; a hydraulic motor having a group ofmotor plungers; a closed hydraulic circuit by which said hydraulic pumpand said hydraulic motor are interconnected; said groups of pump andmotor plungers being disposed in annular patterns in said cylinderblock; and said groups of pump and motor plungers being disposedconcentrically such that one of said groups surrounds the other group,and being positionally displaced in the circumferential direction ofsaid cylinder block such that a circle inscribed in said one grouppasses through said other group; wherein said hydraulic motor is of aswash-plate-type with axial-pistons, said pump and motor each having aplunger swash-plate, said closed hydraulic circuit including high- andlow-pressure oil chambers defined in said cylinder block for supplyingworking oil to and receiving working oil from said plungers in responseto relative rotation of said cylinder block and at least one saidplunger swash-plate, said high- and low-pressure oil chambers beingdisposed annularly around the axis of said cylinder block and extendingaxially of said cylinder block, a plurality of distribution valvesdisposed annularly in said cylinder block concentrically with saidplungers and reciprocally movable between positions spaced axially ofsaid cylinder block for alternately communicating said plungers withsaid high- and low-pressure oil chambers, a holder by which said oneplunger swash-plate is supported, and a valve swash-plate contactingends of said distribution valves for imparting reciprocating motion tothe distribution valves in response to relative rotation of saidcylinder block and said one plunger swash-plate, said valve swash-platebeing supported on said holder obliquely to the axis of said cylinderblock.
 7. A working oil distributor in a swash-plate type hydraulicdevice, comprising:a cylinder block rotatable about an axis, saidcylinder block having an annular array of cylinder holes and a pluralityof plungers slidably disposed respectively in said cylinder holes; aplunger swash plate engaging projecting ends of said plungers; highpressure and low pressure oil chambers defined in said cylinder blockfor supplying working oil to and receiving working oil from saidcylinder holes in response to relative rotation of said cylinder blockand said plunger swash plate, said high pressure and low pressure oilchambers being annularly disposed around the axis of said cylinderblock; and a plurality of distribution valves disposed annularly in saidcylinder block and aligned to reciprocate along a direction parallel tothe axis of said cylinder block for communicating said cylinder holesalternately with said high pressure oil chamber and said low pressureoil chamber, said distribution valves projecting out of said cylinderblock to engage means for reciprocating said distribution valves.
 8. Aworking oil distributor according to claim 7, wherein said means forreciprocating comprises a valve swash plate contacting ends of saiddistribution valves for imparting reciprocating motion to thedistribution valves in response to relative rotation of said cylinderblock and said plunger swash plate.
 9. A working oil distributoraccording to claim 8, wherein said plunger swash plate is supported by aholder and said valve swash plate is supported on said holder obliquelyto the axis of said cylinder block.